The overall purpose of this research is to study the physiological meaning of the phosphorylation of myofibrillar proteins. The working hypothesis is that these phosphorylations may represent controls of cardiac contractility. In order to test this hypothesis, we plan: 1) to determine the effect of agents which modify the contractility of heart, such as adrenergic and cholinergic agonists, on the actomyosin ATPase activity and its control by (Ca2 ion), 2) to study the effect of these agents on the levels of phosphorylation of the different myofibrillar proteins, 3) To identify the specific protein kinases which catalyze these phosphorylations, 4) To study in vitro the effect of the phosphorylation of the myofibrillar proteins, as catalyzed by these kinases, on the actomyosin ATPase. From these studies, we expect to achieve a better understanding of the role of myofibrillar protein phosphorylation on the mechanism of contraction, and to explain the control of these phosphorylations by adrenergic and cholinergic agents.